You should not be building Apache at all, nor should be running an obsolete and unsupported release. 5.8 is the current release. See the CentOS 5.8 Release Notes
for details. By not updating you are implicitly accepting that you will live with numerous bugs and security issues (and associated known exploits) that have subsequently been fixed.
Please read about Installing Software
, and note the warnings and advice about installing and configuring the yum priorities plugin
. Non-core repos should have a higher numeric value, which is a lower priority. Lower is "better" as in golf scores.
Pay particular attention to the evils of Source Installs
. Source installs as root are particularly dangerous.
All you should need to do is to enable the 32-bit repos and install the 32-bit httpd with yum; however, there may be a good reason that the 32-bit package is not in 64-bit repos any more, and rebuilding the library for 64-bit might be a better approach.